Cutaneous vasoconstrictor response to whole body skin cooling is altered by time of day.

To test for a diurnal difference in the vasoconstrictor control of the cutaneous circulation, we performed whole body skin cooling (water-perfused suits) at 0600 (AM) and 1600 (PM). After whole body skin temperature (T(sk)) was controlled at 35 degrees C for 10 min, it was progressively lowered to 32 degrees C over 18-20 min. Skin blood flow (SkBF) was monitored by laser-Doppler flowmetry at three control sites and at a site that had been pretreated with bretylium by iontophoresis to block noradrenergic vasoconstriction. After whole body skin cooling, maximal cutaneous vascular conductance (CVC) was measured by locally warming the sites of SkBF measurement to 42 degrees C for 30 min. Before whole body skin cooling, sublingual temperature (T(or)) in the PM was significantly higher than that in the AM (P < 0.05), but CVC, expressed as a percentage of maximal CVC (%CVC(max)), was not statistically different between AM and PM. During whole body skin cooling, %CVC(max) levels at bretylium-treated sites in AM or PM were not significantly reduced from baseline. In the PM, %CVC(max) at control sites fell significantly at T(sk) of 34.3 +/- 0.01 degrees C and lower (P < 0.05). In contrast, in the AM %CVC(max) at control sites was not significantly reduced from baseline until T(sk) reached 32.3 +/- 0.01 degrees C and lower (P < 0.05). Furthermore, the decrease in %CVC(max) in the PM was significantly greater than that in AM at T(sk) of 33.3 +/- 0.01 degrees C and lower (P < 0.05). Integrative analysis of the CVC response with respect to both T(or) and T(sk) showed that the cutaneous vasoconstrictor response was shifted to higher internal temperatures in the PM. These findings suggest that during whole body skin cooling the reflex control of the cutaneous vasoconstrictor system is shifted to a higher internal temperature in the PM. Furthermore, the slope of the relationship between CVC and T(sk) is steeper in the PM compared with that in the AM.     

Hypohydration effect on finger skin temperature and blood flow during cold-water finger immersion.

This study was conducted to determine whether hypohydration (Hy) alters blood flow, skin temperature, or cold-induced vasodilation (CIVD) during peripheral cooling. Fourteen subjects sat in a thermoneutral environment (27 degrees C) during 15-min warm-water (42 degrees C) and 30-min cold-water (4 degrees C) finger immersion (FI) while euhydrated (Eu) and, again, during Hy. Hy (-4% body weight) was induced before FI by exercise-heat exposure (38 degrees C, 30% relative humidity) with no fluid replacement, whereas during Eu, fluid intake maintained body weight. Finger pad blood flow [as measured by laser-Doppler flux (LDF)] and nail bed (T(nb)), pad (T(pad)), and core (T(c)) temperatures were measured. LDF decreased similarly during Eu and Hy (32 +/- 10 and 33 +/- 13% of peak during warm-water immersion). Mean T(nb) and T(pad) were similar between Eu (7.1 +/- 1.0 and 11.5 +/- 1.6 degrees C) and Hy (7.4 +/- 1.3 and 12.6 +/- 2.1 degrees C). CIVD parameters (e.g., nadir, onset time, apex) were similar between trials, except T(pad) nadir was higher during Hy (10.4 +/- 3.8 degrees C) than during Eu (7.9 +/- 1.6 degrees C), which was attributed to higher T(c) in six subjects during Hy (37.5 +/- 0.2 degrees C), compared with during Eu (37.1 +/- 0.1 degrees C). The results of this study provide no evidence that Hy alters finger blood flow, skin temperature, or CIVD during peripheral cooling.     

Effect of cooling and heating on the regional distribution of blood flow in fetal sheep

This is a study on the effect of cooling and heating amniotic fluid on blood flow to fetal tissues and organs. In 8 unanaesthetized, chronically-catheterised fetal sheep (129-137 days gestation) cold or warm water was passed through tubing encircling the fetus in utero and blood flow was measured using the radionuclide-labelled 15 mu spheres. Following cooling for 30 min, amniotic fluid temperature fell 9.6 degrees C to 29.9 +/- 2.1 degrees C (SEM) fetal arterial temperature fell 2.37 degrees C to 37.30 +/- 0.36, and maternal arterial temperature fell 0.53 degrees C to 38.58 +/- 0.16. Blood flow through the fetal skin fell 60% (P less than 0.01) to 13.6 ml/min per 100 g tissue. Blood flow to the brown fat increased 186% (P less than 0.05) to 99.6 ml/min per 100 g. Following warming for 20 min, fetal temperature rose to 40.43 +/- 0.19 degrees C, and skin blood flow did not change significantly relative to initial control period but rose 200% above that during cooling (P less than 0.01). During both cooling and heating, blood flow to the adrenals rose significantly (P less than 0.05) whereas flow to the carcass, brain, kidneys, and placenta was not altered detectably. Continuous sampling of blood from the inferior vena cava during microsphere injection failed to detect any evidence of arterio-venous shunting through the skin at any temperature studied. Overall, the blood flow responses are consistent with a thermoregulatory role for the skin and brown fat in the near-term fetal sheep.     

Stroke volume during exercise: interaction of environment and hydration

Euhydrated and dehydrated subjects exercised in a hot and a cold environment with our aim to identify factors that relate to reductions in stroke volume (SV). We hypothesized that reductions in SV with heat stress are related to the interaction of several factors rather than the effect of elevated skin blood flow. Eight male endurance-trained cyclists [maximal O(2) consumption (VO(2 max)) 4.5 +/- 0.1 l/min; means +/- SE] cycled for 30 min (72% VO(2 max)) in the heat (H; 35 degrees C) or the cold (C; 8 degrees C) when euhydrated or dehydrated by 1.5, 3.0, or 4.2% of their body weight. When euhydrated, SV and esophageal temperature (T(es) 38. 2-38.3 degrees C) were similar in H and C, whereas skin blood flow was much higher in H vs. C (365 +/- 64% higher; P < 0.05). With each 1% body weight loss, SV declined 6.4 +/- 1.3 ml (4.8%) in H and 3.4 +/- 0.4 ml (2.5%) in C, whereas T(es) increased 0.21 +/- 0.02 and 0. 10 +/- 0.02 degrees C in H and C, respectively (P < 0.05). However, reductions in SV were not associated with increases in skin blood flow. The reduced SV was highly associated with increased heart rate and reduced blood volume in both H (R = 0.96; P < 0.01) and C (R = 0. 85; P < 0.01). In conclusion, these results suggest that SV is maintained in trained subjects during exercise in euhydrated conditions despite large differences in skin blood flow. Furthermore, the lowering of SV with dehydration appears largely related to increases in heart rate and reductions in blood volume.     

Face cooling by cold wind in walking subjects

The effects of low to moderate wind speeds on face temperature, thermal and pain sensations while subjects walked on a treadmill during cold exposure were studied in eight healthy men. The purpose of the study was to evaluate further the risk of frostbite at different activity levels. The walking speed was 2.8 km h(-1) and two inclination levels were used, 0 degrees and 6 degrees. The subjects were exposed to -10 degrees C and 0, 1 or 5 m s(-1) wind for 60 min dressed in cold-protective clothing with only the face unprotected. Results from previous experiments with the same subjects standing for 30 min were included in the analysis of the data. Each individual was exposed to all combinations of air velocity and activity level. The exposure to -10 degrees C and the highest wind speed used would carry no risk of frostbite according to the wind chill index. Cold lowered the skin temperature of the face significantly and wind further increased skin cooling. The activity level did not affect forehead and cheek temperatures, but the average nose skin temperature was higher and pain sensations were reduced at a higher work rate. The predicted risk of frostbite in the nose, based on average responses, would thus be less at a higher work rate. However, the results indicate that exercise does not necessarily protect all individuals from frostbite at moderate air speeds, since the nose skin temperature of 25% of the subjects dropped to 0 degrees C at 5 m s(-1) during both standing and walking. Thus the potential individual risk of frostbite in the nose is similar during light exercise and standing. Moreover, the risk of frostbite seems to be underestimated by the wind chill index under the conditions tested in this study.     

Skin-surface cooling elicits peripheral and visceral vasoconstriction in humans.

Skin-surface cooling elicits a pronounced systemic pressor response, which has previously been reported to be associated with peripheral vasoconstriction and may not fully account for the decrease in systemic vascular conductance. To test the hypothesis that whole body skin-surface cooling would also induce renal and splanchnic vasoconstriction, 14 supine subjects performed 26 skin-surface cooling trials (15-18 degrees C water perfused through a tube-lined suit for 20 min). Oral and mean skin temperature, heart rate, stroke volume (Doppler ultrasound), mean arterial blood pressure (MAP), cutaneous blood velocity (laser-Doppler), and mean blood velocity of the brachial, celiac, renal, and superior mesenteric arteries (Doppler ultrasound) were measured during normothermia and skin-surface cooling. Cardiac output (heart rate x stroke volume) and indexes of vascular conductance (flux or blood velocity/MAP) were calculated. Skin-surface cooling increased MAP (n = 26; 78 +/- 5 to 88 +/- 5 mmHg; mean +/- SD) and decreased mean skin temperature (n = 26; 33.7 +/- 0.7 to 27.5 +/- 1.2 degrees C) and cutaneous (n = 12; 0.93 +/- 0.68 to 0.36 +/- 0.20 flux/mmHg), brachial (n = 10; 32 +/- 15 to 20 +/- 12), celiac (n = 8; 85 +/- 22 to 73 +/- 22 cm.s(-1).mmHg(-1)), superior mesenteric (n = 8; 55 +/- 16 to 48 +/- 10 cm.s(-1).mmHg(-1)), and renal (n = 8; 74 +/- 26 to 64 +/- 20 cm.s(-1).mmHg(-1); all P < 0.05) vascular conductance, without altering oral temperature, cardiac output, heart rate, or stroke volume. These data identify decreases in vascular conductance of skin and of brachial, celiac, superior mesenteric, and renal arteries. Thus it appears that vasoconstriction in both peripheral and visceral arteries contributes importantly to the pressor response produced during skin-surface cooling in humans.     

Effect of body tilt on calf muscle performance and blood flow in humans.

To explore the effect of posture on muscle performance, we tested the effects of body tilt angle on the strength, endurance, and fatigue of, and blood flow into, the plantar flexors. Human subjects were fixed to a tilt table that could tilt them from the horizontal (0 degrees ) to upright (90 degrees ) position and enabled force to be applied to a footplate through isometric action of the right calf muscle. In experiment 1, six subjects performed a strength test and graded test (intermittent contractions) to the point of failure at three tilt angles (0, 47, and 90 degrees ). In Experiment 2, seven subjects performed a strength test and constant-force test [70% maximum force (F(max)); intermittent contractions] to the point of failure in the horizontal and three inclined positions (32, 47, and 67 degrees ). In experiment 3, leg blood flow was assessed during constant-force exercise at two intensities (30 and 70% F(max)) and two tilt angles (0 and 67 degrees ) in six subjects. Strength was not affected (P > 0.05) by tilt angle. Time to failure during the graded test was significantly higher at 47 degrees (25.9 +/- 2.0 min) and 90 degrees (25.1 +/- 3.0 min) than 0 degrees (22.2 +/- 2.6 min). Time to failure during the constant-force test was also significantly higher at 32 degrees (7.1 +/- 3.6 min), 47 degrees (8.0 +/- 5.2 min), and 67 degrees (8.6 +/- 5.6 min) compared with 0 degrees (4.0 +/- 2.6 min). When graded or constant-force exercise was performed with arterial flow to the leg eliminated, there were no differences in exercise time between the horizontal and an inclined position. During nonischemic exercise, leg blood flow was significantly higher during exercise in the inclined position. These results demonstrate that head-up tilt improves endurance of the plantar flexors, that this effect occurs in the absence of an effect on strength, and that it depends on an intact peripheral circulation. Moreover, the postural effect on muscle endurance appears to be due to a greater blood flow into the leg, an effect that is established during the initial contractions.     

Thermographic mapping of the skin surface of the head in bald-headed male subjects

Thermal patterns on the skin are related to skin blood perfusion. While knowledge concerning thermal distribution patterns of the face are well established, the thermal distribution patterns of the skin surface of other aspects of the head has only been investigated once previously in a study employing low resolution thermographic equipment [van Dulken, H., van Voss, S.F.C.H, 1971. The normal thermographic patterns of the human head. Acta Neurochir. 23, 247–58.] The purpose of the study was to re-examine the thermal distribution patterns of the surface of the entire head in 12 healthy bald headed male subjects (mean age 39±7.8 (SD) years) using a high definition infrared (IR) thermographic camera. The thermal distribution patterns were monitored using dynamic infrared thermography (DIRT) before, during and after a 2 min local skin cooling procedure with a pair of fans. Infrared thermal images (thermograms) were taken of the superior, anterior, lateral and posterior aspects of the head. The thermal patterns in general followed the anatomical location of the main superficial arteries and veins of the head as described in anatomy books although there were some variations, especially with regard to the forehead and top of the head (superior aspect), ranging from an asymmetrical distribution pattern to a lack of a clear thermal pattern. The thermal patterns in the left and right lateral thermograms in each individual were relatively symmetrical. During the rewarming period the helix, auricular lobule of the auricles and the nose were relatively cold. The frontal thermograms were characterized by cool nasal and cheek areas and a clear warm area surrounding the eyes, especially around the inner canthus.     

Effect of work load on cutaneous vascular response to exercise.

The purpose of the present study was to examine whether intensity of exercise affects skin blood flow response to exercise. For this purpose, six healthy men cycled, in a random order on different days, for 15 min at 50, 60, 70, 80, and 90% of their maximum oxygen consumption (VO2max) at a room temperature of 25 degrees C. At the end of exercise, esophageal temperature (Tes) averaged 37.4 +/- 0.2, 37.7 +/- 0.2, 37.9 +/- 0.2, 38.6 +/- 0.3, and 38.9 +/- 0.4 degrees C (SE) at the 50, 60, 70, 80, and 90% work loads, respectively. At the two highest work loads, no steady state was observed in Tes. Skin blood flow was estimated by measuring forearm blood flow (FBF) with strain-gauge plethysmography and by laser-Doppler flowmetry on the upper back. Both techniques showed that skin blood flow response to rising Tes was markedly reduced at the 90% work load compared with other work loads. At the end of exercise, FBF averaged 7.5 +/- 1.7, 10.7 +/- 3.1, 9.6 +/- 2.1, 11.3 +/- 2.6, and 5.4 +/- 1.3 (SE) ml.min-1.100 ml-1 (P less than 0.01) at the 50, 60, 70, 80, and 90% VO2max work loads, respectively. The corresponding values for Tes threshold for cutaneous vasodilation (FBF) were 37.42 +/- 0.16, 37.48 +/- 0.13, 37.59 +/- 0.13, 37.79 +/- 0.19, and 38.20 +/- 0.22 degrees C (P less than 0.05) at 50, 60, 70, 80, and 90% VO2max, respectively. In two subjects, no cutaneous vasodilation was observed at the 90% work load.(ABSTRACT TRUNCATED AT 250 WORDS)     

Control of heat-induced cutaneous vasodilatation in relation to age

Well matched unacclimatised older (age 55-68, 4 women, 2 men) and younger (age 19-30, 4 women, 2 men) subjects performed 75 min cycle exercise (approximately 40% VO2max) in a hot environment (37 degrees C, 60% rh). Rectal temperature (Tre), mean skin temperature (Tsk), arm blood flow (ABF, strain gauge plethysmography), and cardiac output (Q, CO2 rebreathing) were measured to examine age-related differences in heat-induced vasodilatation. Tre and Tsk rose to the same extent in each group during the exposure. There was no significant intergroup difference in sweat rate (older: 332 +/- 43 ml.m-2.h-1, younger: 435 +/- 49 ml.m-2.h-1; mean +/- SEM). However, the older subjects responded to exercise in the heat with a lower ABF response which could be attributed to a lower Q for the same exercise intensity. The slope of the ABF-Tre relationship was attenuated in the older subjects (9.3 +/- 1.3 vs 17.9 +/- 3.3 ml.100 ml-1.min-1.degrees C-1, p less than 0.05), but the Tre threshold for vasodilatation was about 37.0 degrees C for both groups. These results suggest an altered control of skin vasodilatation during exercise in the heat in older individuals. This attenuated ABF response appears to be unrelated to VO2max, and may reflect an age-related change in thermoregulatory cardiovascular function.     

Aerobic training and cutaneous vasodilation in young and older men.

To determine the effect and underlying mechanisms of exercise training and the influence of age on the skin blood flow (SkBF) response to exercise in a hot environment, 22 young (Y; 18-30 yr) and 21 older (O; 61-78 yr) men were assigned to 16 wk of aerobic (A; YA, n = 8; OA, n = 11), resistance (R; YR, n = 7; OR, n = 3), or no training (C; YC, n = 7; OC, n = 7). Before and after treatment, subjects exercised at 60% of maximum oxygen consumption (VO2 max) on a cycle ergometer for 60 min at 36 degrees C. Cutaneous vascular conductance, defined as SkBF divided by mean arterial pressure, was monitored at control (vasoconstriction intact) and bretylium-treated (vasoconstriction blocked) sites on the forearm using laser-Doppler flowmetry. Forearm vascular conductance was calculated as forearm blood flow (venous occlusion plethysmography) divided by mean arterial pressure. Esophageal and skin temperatures were recorded. Only aerobic training (functionally defined a priori as a 5% or greater increase in VO2 max) produced a decrease in the mean body temperature threshold for increasing forearm vascular conductance (36.89 +/- 0.08 to 36.63 +/- 0.08 degrees C, P < 0.003) and cutaneous vascular conductance (36.91 +/- 0.08 to 36.65 +/- 0.08 degrees C, P < 0.004). Similar thresholds between control and bretylium-treated sites indicated that the decrease was mediated through the active vasodilator system. This shift was more pronounced in the older men who presented greater training-induced increases in VO2 max than did the young men (22 and 9%, respectively). In summary, older men improved their SkBF response to exercise-heat stress through the effect of aerobic training on the cutaneous vasodilator system.     

Limitations to vasodilatory capacity and .VO2 max in trained human skeletal muscle

To further explore the limitations to maximal O(2) consumption (.VO(2 max)) in exercise-trained skeletal muscle, six cyclists performed graded knee-extensor exercise to maximum work rate (WR(max)) in hypoxia (12% O(2)), hyperoxia (100% O(2)), and hyperoxia + femoral arterial infusion of adenosine (ADO) at 80% WR(max). Arterial and venous blood sampling and thermodilution blood flow measurements allowed the determination of muscle O(2) delivery and O(2) consumption. At WR(max), O(2) delivery rose progressively from hypoxia (1.0 +/- 0.04 l/min) to hyperoxia (1.20 +/- 0.09 l/min) and hyperoxia + ADO (1.33 +/- 0.05 l/min). Leg .VO(2 max) varied with O(2) availability (0.81 +/- 0.05 and 0.97 +/- 0.07 l/min in hypoxia and hyperoxia, respectively) but did not improve with ADO-mediated vasodilation (0.80 +/- 0.09 l/min in hyperoxia + ADO). Although a vasodilatory reserve in the maximally working quadriceps muscle group may have been evidenced by increased leg vascular conductance after ADO infusion beyond that observed in hyperoxia (increased blood flow but no change in blood pressure), we recognize the possibility that the ADO infusion may have provoked vasodilation in nonexercising tissue of this limb. Together, these findings imply that maximally exercising skeletal muscle may maintain some vasodilatory capacity, but the lack of improvement in leg .VO(2 max) with significantly increased O(2) delivery (hyperoxia + ADO), with a degree of uncertainty as to the site of this dilation, suggests an ADO-induced mismatch between O(2) consumption and blood flow in the exercising limb.     

Venous return from distal regions affects heat loss from the arms and legs during exercise-induced thermal loads

To study the role of venous return from distal parts of the extremities in influencing heat loss from the more proximal parts, changes in mean skin temperature (Tsk) of the non-exercising extremities were measured by color thermography during leg and arm exercise in eight healthy subjects. Thirty minutes of either leg or arm exercise at an ambient temperature (Ta) of 20 degrees C or 30 degrees C produced a greatly increased blood flow in the hand or foot and a great increase in venous return through the superficial skin veins of the extremities. During the first 10 min of recovery from the exercise, blood flow to and venous return from the hand or foot on the tested side was occluded with a wrist or ankle cuff at a pressure of 33.3 kPa (250 mm Hg), while blood flow to the control hand or foot remained undisturbed. During the 10-min wrist occlusion, Tsk increased significantly from 28.3 degrees +/- 0.41 degrees C to 30.1 degrees +/- 0.29 degrees C in the control forearm, but remained at nearly the same level (28.0 degrees +/- 0.34 degrees C to 28.2 degrees +/- 0.25 degrees C) in the occluded forearm. In the legs, although Tsk on both sides was virtually identical (32.0 degrees +/- 0.31 degrees C, control vs 32.0 degrees +/- 0.36 degrees C, tested) before occlusion, Tsk on the control side (32.6 degrees +/- 0.27 degrees C) was significantly higher than that on the tested side (32.2 degrees +/- 0.21 degrees C) after ankle occlusion.(ABSTRACT TRUNCATED AT 250 WORDS)     

Are blood flow and lipolysis in subcutaneous adipose tissue influenced by contractions in adjacent muscles in humans?

Aerobic exercise increases whole body adipose tissue lipolysis, but is lipolysis higher in subcutaneous adipose tissue (SCAT) adjacent to contracting muscles than in SCAT adjacent to resting muscles? Ten healthy, overnight-fasted males performed one-legged knee extension exercise at 25% of maximal workload (W(max)) for 30 min followed by exercise at 55% W(max) for 120 min with the other leg and finally exercised at 85% W(max) for 30 min with the first leg. Subjects rested for 30 min between exercise periods. Femoral SCAT blood flow was estimated from washout of (133)Xe, and lipolysis was calculated from femoral SCAT interstitial and arterial glycerol concentrations and blood flow. In general, blood flow and lipolysis were higher in femoral SCAT adjacent to contracting than adjacent to resting muscle (time 15-30 min; blood flow: 25% W(max) 6.6 +/- 1.0 vs. 3.9 +/- 0.8 ml x 100 g(-1) x min(-1), P < 0.05; 55% W(max) 7.3 +/- 0.6 vs. 5.0 +/- 0.6 ml x 100 g(-1) x min(-1), P < 0.05; 85% W(max) 6.6 +/- 1.3 vs. 5.9 +/- 0.7 ml x 100 g(-1) x min(-1), P > 0.05; lipolysis: 25% W(max) 102 +/- 19 vs. 55 +/- 14 nmol x 100 g(-1) x min(-1), P = 0.06; 55% W(max) 86 +/- 11 vs. 50 +/- 20 nmol x 100 g(-1) x min(-1), P > 0.05; 85% W(max) 88 +/- 31 vs. -9 +/- 25 nmol x 100 g(-1) x min(-1), P < 0.05). In conclusion, blood flow and lipolysis are generally higher in SCAT adjacent to contracting than adjacent to resting muscle irrespective of exercise intensity. Thus specific exercises can induce "spot lipolysis" in adipose tissue.     

Effect of pre-cooling, with and without thigh cooling, on strain and endurance exercise performance in the heat

Body cooling before exercise (i.e. pre-cooling) reduces physiological strain in humans during endurance exercise in temperate and warm environments, usually improving performance. This study examined the effectiveness of pre-cooling humans by ice-vest and cold (3 degrees C) air, with (LC) and without (LW) leg cooling, in reducing heat strain and improving endurance performance in the heat (35 degrees C, 60% RH). Nine habitually-active males completed three trials, involving pre-cooling (LC and LW) or no pre-cooling (CON: 34 degrees C air) before 35-min cycle exercise: 20 min at approximately 65% VO2peak then a 15-min work-performance trial. At exercise onset, mean core (Tc, from oesophagus and rectum) and skin temperatures, forearm blood flow (FBF), heart rate (HR), and ratings of exertion, body temperature and thermal discomfort were lower in LW and LC than CON (P<0.05). They remained lower at 20 min [e.g. Tc: CON 38.4+/-0.2 (+/-S.E.), LW 37.9+/-0.1, and LC 37.8+/-0.1 degrees C; HR: 177+/-3, 163+/-3 and 167+/-3 b.p.m.), except that FBF was equivalent (P=0.10) between CON (15.5+/-1.6) and LW (13.6+/-1.0 ml.100 ml tissue(-1) x min(-1)). Subsequent power output was higher in LW (2.95+/-0.24) and LC (2.91+/-0.25) than in CON (2.52+/-0.28 W kg(-1), P=0.00, N=8), yet final Tc remained lower. Pre-cooling by ice-vest and cold air effectively reduced physiological and psychophysical strain and improved endurance performance in the heat, irrespective of whether thighs were warmed or cooled.     

Influence of cooling rate on Saccharomyces cerevisiae destruction during freezing: unexpected viability at ultra-rapid cooling rates

The purpose of this work was to study cell viability as a function of cooling rate during freezing. Cooling rate strongly influences the viability of cells during cold thermal stress. One of the particularities of this study was to investigate a large range of cooling rates and particularly very rapid cooling rates (i.e., faster than 20000 degrees C min (-1)). Four distinct ranges of cooling rates were identified. The first range (A(')) corresponds to very slow cooling rates (less than 5 degrees C min (-1)), and results in high cell mortality. The second range (A) corresponds to low cooling rates (5-100 degrees C min (-1)), at which cell water outflow occurs slowly and does not damage the cells. The third range (B) corresponds to rapid cooling rates (100-2000 degrees C min (-1)), at which there is competition between heat flow and water flow. In this case, massive water outflow, which is related to the increase in extracellular osmotic pressure and the membrane-lipid phase transition, can cause cell death. The fourth range (C) corresponds to very high cooling rates (more than 5000 degrees C min (-1)), at which the heat flow is very rapid and partially prevents water exit, which seems to preserve cell viability.     

Influence of female reproductive hormones on local thermal control of skin blood flow.

Progesterone and estrogen modify thermoregulatory control such that, when both steroids are elevated, body temperature increases and the reflex thermoregulatory control of cutaneous vasodilation is shifted to higher internal temperatures. We hypothesized that the influence of these hormones would also include effects on local thermal control of skin blood flow. Experiments were conducted in women in high-hormone (HH) and low-hormone (LH) phases of oral contraceptive use. Skin blood flow was measured by laser-Doppler flowmetry, and local temperature (T(loc)) was controlled over 12 cm(2) around the sites of blood flow measurement. T(loc) was held at 32 degrees C for 10-15 min and was then decreased at one site from 32 to 20 degrees C in a ramp over 20 min. Next, T(loc) was increased from 32 to 42 degrees C in a ramp over 15 min at a separate site. Finally, T(loc) at both sites was held at 42 degrees C for 30 min to elicit maximum vasodilation; data for cutaneous vascular conductance (CVC) are expressed relative to that maximum. Whole body skin temperature (T(sk)) was held at 34 degrees C throughout each study to minimize reflex effects from differences in T(sk) between experiments. Baseline CVC did not differ between phases [8.18 +/- 1.38 (LH) vs. 8. 41 +/- 1.31% of maximum (HH); P > 0.05]. The vasodilator response to local warming was augmented in HH (P < 0.05, ANOVA). For example, at T(loc) of 40-42 degrees C, CVC averaged 76.41 +/- 3.08% of maximum in HH and 67.71 +/- 4.43% of maximum in LH (P < 0.01 LH vs. HH). The vasoconstrictor response to local cooling was unaffected by phase (P > 0.05). These findings indicate that modifications in cutaneous vascular control by female steroid hormones include enhancement of the vasodilator response to local warming and are consistent with reports of the influence of estrogen to enhance nitric oxide-dependent vasodilator responses.     

Differences in finger skin contact cooling response between an arterial occlusion and a vasodilated condition.

To assess the presence and magnitude of the effect of skin blood flow on finger skin cooling on contact with cold objects against the background of circulatory disorder risks in occupational exposures, this study investigates the effect of zero vs. close-to-maximal hand blood flow on short-term (< or =180 s) skin contact cooling response at a contact pressure that allows capillary perfusion of the distal pulp of the fingertip. Six male volunteers touched a block of aluminium with a finger contact force of 0.5 N at a temperature of -2 degrees C under a vasodilated and an occluded condition. Before both conditions, participants were required to exercise in a hot room for > or = 30 min for cutaneous vasodilation to occur (increase in rectal temperature of 1 degrees C). Under the vasodilated condition, forearm blood flow rate rose as high as 16.8 ml.100 ml(-1).min(-1). Under the occluded condition, the arm was exsanguinated, after which a blood pressure cuff was secured on the wrist inducing arterial occlusion. Contact temperature of the finger pad during the subsequent cold contact exposure was measured. No significant difference was found between the starting skin temperatures for the two blood flow conditions, but a distinct difference in shape of the contact cooling curve was apparent between the two blood flow conditions, with Newtonian cooling observed under the occluded condition, whereas a rewarming of the finger skin toward the end of the exposure occurred for the vasodilated condition. Blood flow was found to significantly increase contact temperature from 40 s onward (P < 0.01). It is concluded that, at a finger contact force compatible with capillary perfusion of the finger pad ( approximately 0.5 N), circulating blood provides a heat input source that significantly affects finger skin contact cooling during a vasodilated state.     

Effects of marked hyperthermia with and without dehydration on VO(2) kinetics during intense exercise.

This study determined whether marked hyperthermia alone or in combination with dehydration reduces the initial rate of rise in O(2) consumption (VO(2) on-kinetics) and the maximal rate of O(2) uptake (VO(2 max)) during intense cycling exercise. Six endurance-trained male cyclists completed four maximal cycle ergometer exercise tests (402 +/- 4 W) when euhydrated or dehydrated (4% body wt) with normal (starting esophageal temperature, 37.5 +/- 0.2 degrees C; mean skin temperature, approximately 31 degrees C) or elevated (+1 and +6 degrees C, respectively) thermal strain. In the euhydrated and normal condition, subjects reached VO(2 max) (4.7 +/- 0.2 l/min) in 228 +/- 34 s, with a mean response time of 42 +/- 2 s, and fatigued after 353 +/- 39 s. Hyperthermia alone or in combination with dehydration reduced mean response time (17-23%), VO(2 max) (16%), and performance time (51-53%) (all P < 0.01) but did not alter the absolute response time (i.e., the time to reach 63% response in the control trial, 3.2 +/- 0.1 l/min, 42 s). Reduction in VO(2 max) was accompanied by proportional decline in O(2) pulse and significantly elevated maximal heart rate (195 vs. 190 beats/min for hyperthermia vs. normal). Preventing hyperthermia in dehydrated subjects restored VO(2 max) and performance time by 65 and 50%, respectively. These results demonstrate that impaired high-intensity exercise performance with marked skin and internal body hyperthermia alone or in combination with dehydration is not associated with a diminished rate of rise in VO(2) but decreased VO(2 max).     

Vascular and metabolic response to cycle exercise in sedentary humans: effect of age

We measured leg blood flow (LBF), drew arterial-venous (A-V) blood samples, and calculated muscle O(2) consumption (VO(2)) during incremental cycle ergometry exercise [15, 30, and 99 W and maximal effort (maximal work rate, WR(max))] in nine sedentary young (20 +/- 1 yr) and nine sedentary old (70 +/- 2 yr) males. LBF was preserved in the old subjects at 15 and 30 W. However, at 99 W and at WR(max), leg vascular conductance was attenuated because of a reduced LBF (young: 4.1 +/- 0.2 l/min and old: 3.1 +/- 0.3 l/min) and an elevated mean arterial blood pressure (young: 112 +/- 3 mmHg and old: 132 +/- 3 mmHg) in the old subjects. Leg A-V O(2) difference changed little with increasing WR in the old group but was elevated compared with the young subjects. Muscle maximal VO(2) and cycle WR(max) were significantly lower in the old subjects (young: 0.8 +/- 0.05 l/min and 193 +/- 7 W; old: 0.5 +/- 0.03 l/min and 117 +/- 10 W). The submaximally unchanged and maximally reduced cardiac output associated with aging coupled with its potential maldistribution are candidates for the limited LBF during moderate to heavy exercise in older sedentary subjects.